RE: [PATCH 1/2] spi: renesas,rzv2m-csi: Add SPI Slave related properties

[Fabrizio Castro]

Hi Geert,

Thanks for your reply!

> From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Subject: Re: [PATCH 1/2] spi: renesas,rzv2m-csi: Add SPI Slave related
> properties
> 
> Hi Mark,
> 
> On Wed, Sep 27, 2023 at 11:00 AM Mark Brown <broonie@xxxxxxxxxx> wrote:
> > On Wed, Sep 27, 2023 at 09:59:05AM +0200, Geert Uytterhoeven wrote:
> > > On Tue, Sep 26, 2023 at 11:08 PM Fabrizio Castro
> > > > +    type: boolean
> > > > +    description:
> > > > +      Use CSI Slave Selection (SS) pin to enable transmission
> and reception when
> > > > +      in slave mode.
> >
> > > Can't this be done in a more generic way? I had expected that the
> > > existing SPI_NO_CS flag can be set using a property in the "slave"
> subnode,
> > > but apparently there is no "spi-no-cs" property defined yet.
> >
> > The description is clearly saying there is a chip select, _NO_CS
> seems
> > entirely inappropriate.  It's not specified in the device tree
> because
> > when there's no chip select for a device it's a fundamental property
> of
> > how the device is controlled and we don't need any information
> beyond
> > the compatible.
> 
> In host mode, it indeed doesn't matter, as you can have only a single
> device connected with SPI_NO_CS.
> In device mode, the device needs to know if it must monitor the chip
> select line or not.
> 
> In hindsight, I should have kept the question I had written initially,
> but deleted after having read the documentation for the corresponding
> RZ/V2M register bits:
> 
>     What does it mean if this is false? That there is no chip select?

Yes, that's what it means. The IP would just use the clock line to
shift data in and out.
Clearly, that could lead to "misunderstandings" as changing
the active level of the clock line on the host side may lead to 
extra clock cycles interpreted by the slave.

> 
> So "spi-no-cs" would be the inverse of "renesas,csi-ss".

Yes, exactly.

Cheers,
Fab

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 --
> geert@xxxxxxxxxxxxxx
> 
> In personal conversations with technical people, I call myself a
> hacker. But
> when I'm talking to journalists I just say "programmer" or something
> like that.
>                                 -- Linus Torvalds